The present invention is directed, in general, to the production of rubber-modified monovinyl aromatic polymers, and more specifically, to a solvent exchange column and method of using the solvent exchange column in the production of rubber-modified monovinyl aromatic polymers.
Methods and apparatus for continuously producing polybutadiene and for producing high impact polystyrene using polybutadiene as the rubber component, are both well known. The solution of polymerization of butadiene to polybutadiene in a hydrocarbon solvent is disclosed, for example in U.S. Pat. Nos. 4,271,060; 4,375,534; 4,495,028; and 4,686,086. According to conventional methods, polybutadiene is made by polymerizing butadiene to about 12 weight percent solids in hexane, butane, cyclopentane, or another hydrocarbon solvent; concentrating the mixture to about 30 weight percent solids by flashing off solvent; steam stripping to remove additional solvent, reduce stickiness and precipitate the crumb rubber; squeezing and drying to remove excess moisture; and agglomerating the dried crumb rubber by the addition of heat to produce regularly shaped bales. The baled rubber is then sent to intermediate storage or transported to plant sites for use in making other products such as rubber-modified polymers.
One widely used rubber-modified polymer is high impact polystyrene (HIPS). HIPS is made by polymerizing styrene monomer having dissolved in it from about 4 to about 15 percent by weight styrene-butadiene rubber (SBR) or polybutadiene rubber. Both are commonly produced with Mooney viscosities of either 35 or 55, and polybutadiene is generally less expensive than SBR. A conventional method for making HIPS using polybutadiene is disclosed, for example, in U.S. Pat. No. 4,777,210.
In certain applications, it may be advantageous to replace bales of polybutadiene with a continuous polybutadiene/styrene mixture that can be fed directly to a conventional HIPS production line, which can substantially reduce the labor, equipment, transportation, storage and energy costs typically associated with the use of polybutadiene bales. In such systems, butadiene is dissolved in an appropriate solvent, after which a catalyst is added to cause the butadiene to polymerize and form a rubber solution. The rubber solution solvent is then exchanged with styrene in a front-end process to form a polybutadiene/styrene feed mixture. This polybutadiene/styrene feed mixture is then transferred to the HIPS plant for conversion to a high impact polystyrene. A significant problem may arise in such systems, however. In order to dissolve the butadiene, a hydrocarbon solvent such as hexane or butane is added. The use of such solvents, however, may significantly reduce polystyrene production rates due to the inert characteristics of the solvents. Thus, in these continuous feed systems, it would be highly desirable to remove as much of the solvent as possible from the front-end process.
Accordingly, what is needed in the art is an apparatus for exchanging the hydrocarbon diluent with a monovinyl aromatic polymerizable monomer and a method of making a high impact polystyrene using such an apparatus.
To address the above-discussed deficiencies of the prior art, the present invention provides a solvent exchange column for continuously exchanging styrene monomer with the solvent used to produce polybutadiene and a method of use therefor. The solvent exchange column includes an exchange plate support structure coupled to and supporting spaced apart exchange plates, wherein each of the exchange plates has holes located therein to allow a passage of fluid therethrough. The solvent exchange column further includes a vapor ventilation system that extends through the solvent exchange column and allows a volatilized substance to pass therethrough. In addition, in a preferred embodiment, the vapor ventilation system may also provide an overflow port to further prevent a substantial build-up of fluid on the exchange plates.
Another aspect of the present invention provides a method for using the solvent exchange column. The method, in one exemplary embodiment, may include (1) passing a mixture, including a solvent, through exchange plates of a solvent exchange column, the exchange plates having holes located therein to allow a passage of mixture therethrough, (2) volatilizing a substantial portion of the solvent from the exchange plates, and (3) venting a substantial portion of the solvent through a vapor ventilation system of the solvent exchange column.
An alternative aspect of the present invention provides a system for preparation of a rubber-modified monovinyl aromatic polymer. The system, may include a reactor for forming a mixture of a monovinyl aromatic monomer, an unsaturated hydrocarbon polymer having the general formula CnH2nxe2x88x922 and a solvent, the solvent exchange column described above coupled to the reactor, and a styrene polymerization reactor coupled to the solvent exchange column.
The solvent exchange column and method of use therefor, and the system for preparation of a rubber-modified monovinyl aromatic polymer, are believed to be useful for easily and inexpensively producing HIPS containing high-quality polybutadiene rubbers. Of equal importance, is the ability to do this while obtaining the highest production rate of polystyrene as possible. To obtain such high rates, it is important to remove as much of the solvent as possible, that is used to polymerize the butadiene. The solvent exchange column, as provided by the present invention, is capable of recovering substantial amounts of the solvent and, consequently, provides a purer feed for the HIPS plant from which increased production rates of high impact polystyrene may be obtained. Those knowledgeable in the art will recognize the fact that polybutadiene solutions in hydrocarbon solvents are very viscous solutions capable of forming crosslinked polymers at certain temperatures; thus, the solvent exchange column incorporates a novel design to prevent gelation during the solvent exchange operation and homopolymerization of styrene.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.